In this paper,an off axis three mirror optical system is designed using freeform surface.There are three off axis mirrors in the optical system,the second mirror adopts freeform surface,and the rest are with asphere surface.The aperture of the optical system is Φ100mm,and the F/# is 2.5.The system is free of chromatic aberration because of using the reflecting optics.For the optics and mount are made of the same material,the optical system is athermalization.The system off axis aberrations are well corrected by using the non rotational symmetry of free form surface.Thus the optical system is diffraction limited.By properly layout of the three off axis mirrors,the dimension of the optical system reaches 160mm×110mm×250mm.